Vehicle seats with all-belts-to-seat seat belt mechanisms are becoming increasingly important because of the simplification in vehicle design that these mechanisms allow. The seat belt mechanism restrains the occupant during impact during a collision. In an all-belts-to-seat design the seat belt mechanism is integral with the vehicle seat. This type of seat belt mechanism once adjusted to the comfort of the occupant does not require readjustment when the seat as a whole is moved. All-belts-to-seat mechanisms allow for simplification of automobile design because the structural requirements of the B-pillar are reduced such that the need for the B-pillar becomes superfluous (convertibles lack B-pillars). The B-pillar is the section of the vehicle frame that runs between the front and rear windows. However, the structural requirements of the seat increase. The seat must be sufficiently rigid to withstand the forces generated during a collision. Previous attempts to provide the requisite strength have fortified the vehicle seat with various metals or rigid composites. Other attempts to strengthen vehicle seats have done such by increasing the seat component dimensions. These approaches tend to increase the weight of the vehicle which is undesirable.
Currently, most commercial vehicles utilize a three point seat beat mechanism. In the typical three point mechanism, a seat belt retractor pays out seat belt webbing that passes through a D-ring that is attached to the B-pillar of the vehicle frame. The section of the seat belt webbing that runs from the D-ring diagonally across the occupant is usually referred to as the shoulder belt. The shoulder belt attaches to a point on the opposite side of the occupant. From this point of attachment, the seat belt seat belt webbing travels across the occupant to a point of attachment on the opposite side. This latter section of webbing is usually referred to as the lap belt. These last two points of attachment are typically made to the vehicle body floor. Three point seat belt mechanisms unevenly disperse the forces that are developed during a collision by creating a greater force exerted on the side of the occupant closest to the B-pillar. The uneven dispersion of the impact forces creates a twisting motion of the occupant about the B-pillar. During collision only a small amount of the seat belt webbing is paid out.
A four point seat belt mechanism is more desirable than the standard three point seat belt mechanisms because in such a mechanism forces created during a collision are more symmetrically dispersed about the vehicle seat. In a four point mechanism, two points of seat belt attachment are located on both sides of the vehicle occupant. More symmetrical dispersion of the impact forces has less tendency to cause a twisting motion and less accumulation of the force on any given side of the occupant. Thus the four point design is more suitable for vehicles without B-pillars, such as convertibles and vans which have seats that swivel. But with the four point design there is a problem. The seat back frame needs to be sufficiently strong to absorb and disperse the impact forces, and to provide a sufficient cavity for housing the seat belt mechanism.